1. Field of the Invention
The present invention relates to a silicon carbide ceramic and a honeycomb structure. More specifically, the invention relates to a silicon carbide ceramic having a small amount of resistivity change due to temperature change and being capable of generating heat by current application, and a honeycomb structure using such a silicon carbide ceramic as a material thereof.
2. Description of Related Art
Silicon carbide is a compound semiconductor with good conductivity and has an excellent thermal resistance and chemical stability. Therefore, silicon carbide has been utilized as a current application heating element to be used in high-temperature electric furnaces and the like. Generally, silicon carbide exhibits the behavior that “the resistivity rapidly decreases and it converts to increase at about 400° C. as a minimal” with a temperature increase due to current application heat generation. This has been considered on the basis that silicon carbide is a semiconductor. That is, since silicon carbide is a semiconductor, the number of conduction electrons capable of being excited from an impurity level to a conduction band increases with a temperature increase. The resistivity decreases within a range from normal temperature to about 400° C. by this behavior. At temperatures exceeding about 400° C., since the mobility of conduction electrons decreases due to thermal vibration of lattice, it has been considered that the resistivity exhibits a slightly increasing tendency.
Thus, the silicon carbide exhibits the negative property that the resistivity change depending on temperature is a negative (property of showing a decrease in resistivity with a temperature increase) within a range of from normal temperature to about 400° C. Therefore, there were following problems in the case of using silicon carbide as a heating element and increasing the temperature from the normal temperature to about 400° C. by current application. That is, since the resistivity of the silicon carbide (heating element) decreases with the above-mentioned temperature increase, and an electric current may be rapidly increased. In addition, it is very difficult for a heating element exhibiting a large change rate in the above-mentioned “resistivity change depending on temperature” {100×(magnitude of a change in resistivity)/(magnitude of a temperature chancre)} to control the temperature.
In contrast to this, there has been proposed a silicon carbide heating element being made of a “silicon carbide sintered body containing at least 10% of β-SiC crystal particles and having nitrogen dissolved therein” in attempt to reducing the resistivity change depending on temperature (see, for example, Patent Document 1). There has been also proposed a conductive silicon carbide ceramic material composed mainly of “silicon carbide containing 90% or less of 6H—SiC with respect to the entire crystal system and having nitrogen dissolved therein” and having a predetermined temperature coefficient of resistance (see, for example, Patent Document 2).